One of the most common types of escapement mechanisms using a movable-frame motor is the so-called inverted-anchor type. In such an arrangement two principal difficulties are encountered which are not normally seen in normal anchor-type escapements. First of all gravity cannot be relied on to hold the anchor in a particular position so that means must be provided to do this. In addition it is possible for the escape wheel to make false steps, that is moving a half step forward then a half step backward rather than a full step forward on each rocking of the anchor. This false-stepping frequently occurs when the watch is subjected to a shock at the stepping instant as for example when the date changes or the user makes a brusque movement.
It has been suggested to overcome the first-mentioned difficulty by providing a small permanent magnet in the watch which holds the anchor in either of its positions with a pallet engaged between the teeth of the escape wheel. The second problem has been overcome by using a unidirectional spring-type clutch on the escape wheel. It has also been suggested to use an escapement with movable pallets which are displaceable in a plane parallel to the plane of the anchor, their displacement being limited by appropriate means so as to hold the escapement wheel even during the stepping operation.
Another system is known wherein a movable bar carrying two pin-type pallets is used to operate the escape wheel. The bar is reciprocated back and forth by an arm, each movement of the arm moving the escape wheel by a single step. Such an arrangement cannot be driven by a balanced motor, a rotating motor, or an oscillating motor in a timepiece. In addition such a system is relatively sensitive to shocks so as to be almost useless in a portable timepiece. See U.S. Pat. No. 637,179.
It is also possible to operate the above-mentioned bar with a cam as is employed in some self-winding mechanisms. See Swiss Pat. No. 342,302. In such systems, however, the driving cam always remains in contact with the edge of the opening that it cooperates with so that the escapement is always in contact with the drive motor. Such contact destroys the isochronism of the watch and therefore is only employable in timepieces not expected to have a high degree of accuracy. When a rotating or oscillating motor is used the rubbing cam gives relatively poor results.